1. Field of the Invention
The invention relates to a high-frequency circuit for a radio apparatus.
2. Related Art of the Invention
A frequency synthesizer in which loop filters are switched in accordance with the oscillation frequency of a VCO (Voltage Controlled Oscillator) has been proposed in U.S. patent application Ser. No. 682,255. In the synthesizer, a signal obtained by dividing the oscillation frequency of the VCO by a frequency divider, and an output signal of a reference oscillator are input to a phase comparator, and an error signal corresponding to the phase difference between the two signals is input to the VCO through the loop filter as a control signal. In this way, a feedback loop is formed so that an output signal is obtained at the same accuracy as that of the reference signal. The oscillation frequency is changed by changing the division ratio of the frequency divider. According to the invention of the above-mentioned patent application, when the oscillation frequency is changed, loop filters can be switched in accordance with the changed oscillation frequency, thereby enabling the synthesizer to always output a signal of a high accuracy.
However, the proposed synthesizer has several points which remain to be improved. First, a case will be considered where a VCO in which output oscillation frequency bands can be switched by an external control signal is used. In this case, loop filters must be switched so as to conform not only to the frequency obtained as a result of the switching operation and also to the sensitivity of the VCO attained after the switching operation. Next, a case will be considered where the synthesizer is provided with a frequency modulation function using a digital modulation signal. In this case, particularly when a modulation signal of a high bit rate is used, modulation cannot be performed while the feedback loop of the synthesizer is closed. Also in this case, modulation must be performed by using the open-loop modulation system in which the feedback loop is opened only during the modulation process. In the case of using the open-loop modulation system, however, the open-loop modulation cannot satisfactorily be performed by a filter which conforms only to a mere frequency change. In order to satisfactorily perform the open-loop modulation, the VCO must be designed so as to exhibit a small frequency change even in an open loop state.